1. Field of the Invention
The present invention relates generally to improvements in membrane dome switches of the type having one or more conductive spring members adapted for selective operation in response to fingertip pressure applied to a graphics overlay membrane, and more particularly to an improved membrane dome switch designed for enhanced and consistent tactile feedback sensation in response to fingertip switch depression.
Membrane dome switches are generally known in the art, particularly for use in operating a variety of electronic products, including hand-held and desktop devices such as calculators, computers, programmers, and the like. Such membrane dome switches typically comprise a conductive dome-shaped spring disk retained in a sandwich array between an underlying circuit layer having a conductive circuit pattern formed thereon, and an overlying shield layer. Fingertip pressure applied to predetermined points or regions of the shield layer is effective to deform the spring disk in a manner contacting the circuit layer, thereby achieving momentary closure of a circuit path for purposes of operating the electronic device.
In most designs, the overlying shield layer is covered in turn by a graphics overlay membrane or display panel which has appropriate operational instructions and/or indicia to facilitate fingertip depression of one or more dome switches. The graphics overlay membrane and the shield layer are sufficiently flexible in a direction perpendicular to the planes thereof to accommodate relatively easy switch operation.
Although membrane dome switches have enjoyed widespread use in modern electronic products, inconsistent tactile feedback sensation during switch operation can result in difficulties and/or frustrations in manipulating the dome switches to operate the electronic device. That is, the domed spring disks are normally maintained in correct alignment between the shield and circuit layers by positioning the spring disks within apertures formed in a retainer layer sandwiched between the circuit and shield layers. Variations in spring disk height as a result of normal manufacturing tolerances, relative to the thickness of the retainer layer, can have an adverse impact on tactile feedback fingertip sensation. Such tactile feedback is, of course, highly desirable since it provides a user with positive confirmation of switch depression.
More specifically, a spring disk which is too short relative to the thickness of the retainer layer can require a substantial downward force to be applied to the graphics membrane and shield layer in order to deform the spring disk sufficiently to close a circuit path. By contrast, when the spring disk is too tall relative to the retainer layer, the spring disk can be preloaded by the shield layer with the result that the spring disk operation can be inconsistent. In either case, dimensional variations on the order of a few thousandths of an inch can cause tactile feedback sensation to be significantly reduced or lost altogether.
There exists, therefore, a significant need for improvements in and to membrane dome switches of this general type. It is accordingly the primary objective of the present invention that it provide a membrane dome switch having a positive and easily detected tactile feedback sensation which is provided to the user notwithstanding normal dimensional variations present in domed spring disks. It is also an objective that the advantages and objectives of the membrane dome switch of the present invention be achieved without incurring any substantial relative disadvantage.